There are known, in particular from marine applications, fuel vapour extraction systems for an internal combustion engine that is supplied with a volatile liquid fuel from a fuel storage tank and that has an air intake system and a liquid fuel injection system for dispensing fuel to mix with air to be burnt in the engine. The fuel vapour extraction system includes a volatising chamber connected to the fuel storage tank by a valve serving to maintain a constant liquid level of fuel in the chamber and a vapour space above the liquid level in the chamber, and means for drawing vapour from the vapour space in order to maintain a reduced pressure in the volatising chamber. The fuel injection system includes a fuel circulation pump for drawing liquid fuel from the volatising chamber and supplying the fuel under pressure to a fuel rail, fuel injectors for dispensing metered quantities of fuel from the fuel rail to the engine cylinders, a relief valve for maintaining a constant fuel pressure in the fuel rail and a fuel return pipe for returning unused fuel from the fuel rail to the volatising chamber. Examples of such systems are to be found in U.S. Pat. Nos. 5,647,331, 5,115,784 and 5,579,740 and in WO89/06312.
Such systems are employed in marine applications because safety regulations relating to marine vessels in some countries forbid the return of fuel from the injection system to the main fuel storage tank. Instead the fuel is returned to a separate chamber and steps are taken to extract vapour from the latter chamber to avoid vapour lock in the fuel. These systems do not intentionally fraction the fuel to enable the engine management system to make the best use of the different fractions.
U.S. Pat. No. 5,373,825 discloses a fuel vapour extraction system intended for an engine burning a heavy oil that comprises a volatising chamber separate from the fuel tank for intentionally volatising the lighter fraction of the oil. Within the volatising chamber, the oil is heated by a heating element and the lighter fraction of the oil is driven out by the applied heat at substantially ambient air pressure, ambient air being admitted into the chamber to mix with the gasified fuel-and transport it to the air supply of the engine. The remaining liquid fraction that is not gasified is also drawn from the volatising chamber by a fuel injection system and injected into the combustion chamber of the engine. Only a small quantity of surplus oil, that is not injected into the engine, is recycled to the volatising chamber.
This vapour extraction system has the advantage of achieving a continuous supply of fuel vapour, the availability of which can be used to advantage by a suitably designed engine operating with vaporised fuel. However, to achieve a continuous vapour supply, heating energy must be applied continuously to heat the oil which is a drain on the fuel consumption of the engine. The heating element raises the temperature of the oil to a point where the lighter fraction begins to boil and further heating then provides the latent of heat of vaporisation for maintaining a steady gasifying rate.
If the invention is applied to gasoline fuel instead of a heavy oil, the remaining liquid fraction that is drawn from the volatising chamber will be too hot and will need to be cooled before it is delivered to the fuel injection system in order to avoid vapour lock in the fuel injection system. If the bulk of the fuel is recirculated, as occurs in gasoline engines under idle and low load conditions, the system becomes very wasteful of energy as the same fuel is repeatedly heated and then cooled, which is reflected in high fuel consumption.
There are several other disadvantages associated with the application of external heat to the fuel in the volatising chamber. For example, during cold start, there will not be available an adequate supply of vapour until the temperature of the fuel has been raised sufficiently. Furthermore, because of the slow response of the heating element, it will not be possible to increase the vapour flow rapidly when there is sudden increase in the demand for vapour. Also, because of the limited rate at which the heated fuel can be cooled, it will not then be possible to cool the increased flow of the hot liquid fuel sufficiently rapidly before it enters the fuel injection system, thereby risking vapour lock.